My particles are vanishing after 250 frames, not sure whats going on. (Lifetime is set to 350)
Did you modify any of the settings for the second time? Like did first you set lifetime to 250 and later to 350? It that's the case, you should set current frame to 0 and replay it with AltA

Another possibility is the clipping: set the End value of clipping to a higher value or start value to a lower value (you can tab the panel with shortcut N